This invention relates to sensor arrays, and especially to passive optical sensor arrays that are located in environments in which the sensor array is difficult to access. The invention is particularly suitable for undersea seismic sensor arrays, although it will be appreciated that the invention may be employed with sensors of other types. For example, the array may be employed with electric field sensors for determining the presence of oil by changes in the electric field as the conductivity of the rock that contains the oil changes. In other systems, the array could be part of a security warning system that contains a number of hydrophones for detecting unauthorised vessels.
Undersea seismic sensor arrays are widely used in the exploration of and monitoring of oil and gas reservoirs beneath the seabed. In these seismic monitoring techniques, an array of accelerometers and/or hydrophones are deployed as sensor packages on the seabed and are used to detect reflected seismic waves, and the results are analysed to provide information relating to the nature and state of geological structures beneath the seabed.
Typically a large number of sensors, for example 16,000 or more, are arranged along a number of optical cables that are spaced apart from one another to form a two-dimensional array that extends over a large area for example an area of 100 square kilometers or more. In one form of arrangement which may be referred to as a “4C” sensor unit, three seismic vibration sensors are arranged in orthogonal directions together with one hydrophone to form an optical sensing unit (OSU), and a number of optical sensing units are located along an optical line at spaced apart intervals, for example in the range of from 20 to 100 meters. A number of lines, for example 30 although more or fewer may be employed, may extend from a hub located on the seabed in a direction generally parallel to one another and spaced apart from one another, for example by from 100 to 500 meters, to form the array. The hub may be connected by an optical cable to an interrogator located on an exploration or production platform or on a floating production and storage offloading vessel (FPSO) that monitors the sensors by reflectometry or other interferometric means. The optical cable will contain at least one optical fibre for each of the lines that extend from the hub (typically one fibre pair). In operation, the interrogator sends an optical pulse along the cable where it is split at the hub before being sent along the individual lines to the optical sensor units. The vibration sensors may comprise a length of optical fibre that is wound around a flexible former to form a coil, and the optical lines may contain reflectors, for example formed by a mirror that terminates a fibre spliced with the line, preferably upstream and downstream of the sensors. As the external pressure varies, the coil of fibre is compressed or released, thereby changing the length of fibre in the coil. If a signal is sent along the optical fibre, it is partially reflected back along the line at each of the mirrors so that the signal, for example a phase shift in the signal that is dependent on the distance between the reflectors, is affected by any seismic activity. In this way, any mechanical impulse caused by an air gun or other explosion in the vicinity of the array will cause a phase change in signals reflected by the sensors in the array which may be observed by the interrogator.
The signals that are sent along the optical lines will normally be multiplexed in view of the large number of sensor units, usually both time division multiplexed and wavelength division multiplexed.
The interrogator of the system thus typically comprises a transmitter having a number of light sources such as lasers, e.g. 16, for forming the optical signals, and optical switches, and a receiver for receiving and processing the reflected optical signals. The receiver will need to demultiplex a number of wavelength and time division multiplexed streams arriving from the various optical lines of the sensor array, convert the optical signals to electrical signals, digitise them and transmit them onwards or store them. The interrogator is normally the only part of the system which contains electronics or requires electrical power.
Such sensor arrays may include a large number of optical fibre pairs, for example 100 to 200 pairs or more depending on the size of the array, and even up to 700 fibres in some cases, and these will extend from the hub to the platform or FPSO in the form of a riser cable which extends generally vertically from the seabed, although there may be a significant horizontal component, whereupon the cable will extend to a receiver unit of the interrogator located on the platform or FPSO.
While such systems generally work well in practice, they can have a number of problems. For example, in some forms of design where the sensor array is a long distance from the interrogator this would require a riser cable with 100 to 200 fibre pairs extending in the region of 100 km or more between the interrogator and the array, which can be impractical and extremely costly. In other circumstances the platform or FPSO may employ existing optical cables for receiving data from the array, in which case it may not have sufficient optical fibres in the riser. For example many installations may employ existing optical cables having only six fibres or so. In yet other instances, it can be difficult to direct the fibres in the cable from the riser to the interrogator and, in many circumstances, such a riser cable termination is not possible. For example, in the case of an FPSO, the riser cable may emerge onto a stationary turntable whereas the rest of the interrogator will be located on the vessel which may rotate about the turntable at least to a limited extent due to tides and currents etc. This will often require some means of allowing the optical fibres to rotate about the axis of the riser cable at least to a limited extent, for example a slip ring otherwise called a fibre optic rotary joint, to allow the optical fibres to extend between the riser cable and the interrogator on the FPSO. However, such slip rings typically only accept a few optical fibres and even the largest number of optical fibres that can be accepted by a slip ring (31 at this time) is only a fraction of the number of optical fibres in a typical riser cable, so that seven slip rings would be required. Furthermore, the specification of such a slip ring is insufficient for the purpose of a seismic optical fibre array in many cases since the two-way insertion loss may be 9 dB bringing the insertion loss of the array above 60 dB in some cases. In addition, the minimum return loss of the slip ring may be 18 dB, which means that back reflections my be sent to the array degrading its performance, or alternatively isolators would be required in order to prevent such back reflections. Finally, the physical size of the interrogator may be quite large, in the order of two or three cubic meters, and there may not be enough space on the platform or FPSO for the interrogator.